When longitudinal waves are reflected from the surface of a rarer medium?

When a longitudinal wave is reflected from a rarer medium, the phase change produced is

When sound waves are reflected from a denser medium

Explain reflection of longitudinal wave travelling from denser to rarer medium.

When a light wave in a rarer medium is reflected from the surface of an optically denser medium, it suffers a phase change of (in radian)

What is phase difference,when longitudinal wave is reflected from rigid wall?

What is phase difference,when longitudinal wave is reflected from rigid wall?